How to get decent sound out of a Contact Microphone
How do piezos work?
Piezo comes from the Greek word “to squeeze”, which is the basis of how they function. Inside of a piezo material, there are electric charges which are fixed relative to the shape of the material. So, when the material is squeezed, the charges move with the material and create a voltage. This can also work in reverse, such that applying a voltage can cause the material to shrink or expand. A representative drawing of this is shown below.
Figure 2 – Piezo material being compressed and realigning internal dipole charges.
Examples of good amplifier circuits are shown above. You will note the 10kohm resistors in series with the amplifier inputs. These limit the in-rush of current in case of large voltage spikes, and help protect the amplifiers. The diodes on the op-amp clamp the voltage to the supply rails. The JFET has an internal diode, as do some op-amps, so external diodes are not strictly necessary. Although the voltage produced by a piezo can be quite large, the current is extremely small, so the 10kohm resistor is usually enough to protect your circuit.
Figure 4 – Example piezo circuits with high-impedance input, and input protection.